This invention relates to the drilling of subterranean wells, such as oil and gas wells. More particularly, this invention relates to a buoyant drill pipe, to a drilling method employing the drill pipe, and to a drilling system incorporating the drill pipe.
Extended reach wells have been drilled with increasing frequency in recent years to recover liquid and gaseous hydrocarbons from subterranean formations. In drilling an extended reach well, a generally vertical well bore is first drilled from the earthen or subsea surface to a depth approximating a subterranean formation of interest. The well bore is then deviated through a curved segment, and terminated in a horizontal segment or an inclined segment.
Depending upon the radius of curvature of the curved segment, the extended reach well is either completed open hole, or a casing is positioned in the vertical and horizontal or inclined segments, and cemented. The casing is then placed in fluid communication with the formation by perforating or other method. Alternately, a horizontal drainhole can be drilled from an existing well bore by milling a portion of the casing in place in the well bore, and then drilling the horizontal drain hole using a conventional drill string and bit.
Extended reach wells are often drilled in offshore fields to reach reservoirs located some distances from an existing platform or from land. In these situations it is usually cheaper to drill an extended reach well from the existing platform, or from land, to reach the additional reservoirs. The extended reach well saves the expense of building separate platforms directly over each reservoir in a field.
In view of these advantages, technology has been developed to facilitate drilling of extended reach wells, (i.e.., wells in which the ratio of the measured depth to the true vertical depth is at least 2.0). This technology is sometimes referred to as extended reach drilling (xe2x80x9cERDxe2x80x9d). Using ERD, wells have been drilled with a maximum closure (i.e., directional reach of horizontal departure) of greater than about 18,000 ft., and a true vertical depth (xe2x80x9cTVDxe2x80x9d) of about 4,500 ft.
One aspect of ERD is that current drilling equipment is limited by the service limits of the drill pipe. Presently, the maximum make up torque of steel drill pipe is about 65,000-70,000 foot/pounds. This limit is usually met at about 20,000 feet of maximum closure, and at about 6,000-6,500 TVD. Depending upon the maximum closure and true vertical depth of an extended reach well, the tensile strength of the drill pipe is often the limiting factor.
Drill string dynamics, such as friction, resulting from the rotation of the drill string by a rotary drive system, can also cause problems in ERD. For example, during rotation, the drill string can encounter resistance to free rotation from cuttings within the well bore, or from long sections of drill pipe rubbing against the well bore. With rotational resistance, higher torque forces must be placed on the drill string by the rotary drive system. Also during rotation, the drill string can wobble, increasing the torsional loads on the drill string.
One prior art approach to the problem of high torque requirements has been to make the drill pipe out of light weight materials, such as aluminum or titanium. The lighter drill pipe makes the drill string lighter, and easier to rotate, thus reducing torsional loads. However, this solution has not been totally satisfactory, as lightweight drill pipe is expensive, and lacks the durability of conventional steel drill pipe.
In other prior art drilling systems, the drill pipe has been made more buoyant by charging the drill pipe with a buoyant gas or fluid. This increased buoyancy reduces the weight of the drill pipe in relation to the column of fluid in which it is suspended, and decreases the rotational forces required to rotate the drill string. However, these prior art systems have not provided completely satisfactory results, particularly for ERD. Thus, a need exists for an improved drill pipe, and for an improved drilling method, in which the weight of the drill string, and torsional stresses on the drill string during drilling are reduced.
Another problem with ERD is that cleaning of formation cuttings from the well bore becomes increasingly difficult in the horizontal and inclined segments of the well. Larger diameter drill pipe has been employed to increase the quantities of drilling fluids flowing in the pipe, to facilitate removal of the cuttings from the well bore. However, such larger diameter drill pipe does not alleviate the problems associated with high torque resistance and drill string dynamics. Accordingly, a need exists for a drill pipe that improves the removal of formation cuttings from horizontal and inclined segments of the well bore.
In view of the foregoing, it is an object of the present invention to provide an improved drill pipe having an increased buoyancy, and which can be rotated with reduced torque and torsional stresses. It is still another object of the present invention to provide an improved drill pipe having an increased outside diameter, which increases the flow rate of drilling fluids, and facilitates cleaning of formation cuttings from the well bore. It is a further object of the present invention to provide an improved drilling method and an improved drilling system that employ a buoyant drill pipe.
In accordance with the present invention, a drill pipe, a drilling system and a drilling method for subterranean wells are provided. The drill pipe, drilling system and drilling method are particularly suited to drilling extended reach wells having horizontal or inclined segments.
The drill pipe, broadly stated, comprises a tubular element, such as a pipe or tube, having one or more buoyant elements attached thereto. The buoyant elements are configured to interact with a drilling fluid in the well bore to provide buoyancy for the drill pipe.
In a first embodiment, the drill pipe includes a tubular element with threaded connections at each end, and a buoyant inflatable element attached to an outside diameter of the tubular element. The threaded connections permit multiple drill pipes to be connected to one another, and to other drilling components, to form a drill string. The drill string provides a conduit for injecting the drilling fluid into the well bore, and also forms a well annulus for returning the drilling fluid to the surface with formation cuttings.
The inflatable buoyant element contains a buoyant fluid, such as a gas or a liquid, which increases the buoyancy of the drill string in the drilling fluid. The increased buoyancy decreases the weight of the drill string in the well bore, reduces the torque required to rotate the drill string, and reduces the rotational stresses on the drill string. This permits well bores with longer horizontal or inclined segments to be drilled. In addition, the inflatable element increases the outside diameter of the drill string, such that the well annulus is constricted, and the flow rate of the drilling fluid in the well annulus is increased. This facilitates removal of formation cuttings from the well bore by the drilling fluid. The drill pipe can also include an outer casing, or other mechanism, for limiting the outside diameter of the inflated inflatable buoyant element.
In a second embodiment, the drill pipe includes a tubular element, and a buoyant element in the form of a buoyant collar attached to an outside diameter of the tubular element. The buoyant collar can be made of a buoyant material, such as plastic, foam, or a composite material. In addition to providing buoyancy, the buoyant collar also reduces frictional forces between the drill string and the well bore. In a third embodiment the drill pipe includes a tubular element, and a buoyant collar having one or more pockets for containing a gas, or a second buoyant material. In a fourth embodiment the drill pipe includes a tubular element in the form of a coiled tube, and multiple buoyant elements attached at spaced intervals to the tubular element.
The drilling system includes the drill string formed by multiple connected buoyant drill pipes. The drilling system also includes a drill bit attached to the drill string, a rotary drive mechanism for rotating the drill string, and a source of a drilling fluid in flow communication with the drill string.
The drilling method, broadly stated, includes the steps of: providing a drill pipe comprising a tubular element and a buoyant element, connecting the drill pipe to a drill bit to form a drill string, rotating the drill string and the drill bit through an earthen formation while injecting a drilling fluid through the drill string into the well bore; and applying a buoyant force to the drill string by interaction of the buoyant element with the drilling fluid in the well bore.